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Physiology & Science: Immunity Basics > Adaptive Immunity II > Flashcards

Adaptive Immunity II Flashcards

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1
Q

What are components of the adaptive immune system?

A
  • T lymphocytes -> cellular immunity
  • B lymphocytes -> humoral immunity
  • Antibodies
  • Cytokines
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2
Q

Classes of T lymphocytes: What are Th (helper) cells?

A
  • Express CD4
  • Activate macrophages
  • Help B cells to produce antibodies
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3
Q

Classes of T lymphocytes: What are CTL (cytotoxic) T cells?

A
  • Express CD8
  • Kill cells infected with microbes
  • Kill tumour cells
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4
Q

Classes of T lymphocytes: What are Treg (regulatory) cells?

A
  • Inhibit function of other T cells
  • Control of immune responses
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5
Q

Where are antigen receptors found?

A
  • On B and T cells
  • Ig on surface of B cells; TCR = T cell receptor
  • Can virtually recognise any microbial structure
  • Distinguish antigens on different microbes and on same microbes
  • When self-non-self discrimination fails -> autoimmune disease
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6
Q

How do both B and T cells recognise antigens (in general terms)?

A
  • B cells recognise antigen directly (as they have antibodies on surface)
  • T cells - the antigen is processed + presented by APC (eg macrophages, dendritic cells)
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7
Q

How do antibodies work?

A
  • Bind to extracellular microbes + toxins
  • neutralise (prevent binding of antigens to receptors or cells ; block antigen entry or effects on cells)
  • eliminate microbes: opsonisation + complement activation
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8
Q

What is meant by intracellular microbes? Examples?

A
  • Cell-mediated immunity fights intracellular microbes
  • Intracellular bacteria in phagosomes of phagocytes
  • viruses: cytoplasm of cells (phagocytes or non-phagocytes eg. epithelial cells)
  • parasites (helminths) - both intra- and extra-cellular
  • Main effectors: T cells
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9
Q

Can B and T cells both recognise soluble and cell-bound antigens?

A
  • B cells recognise soluble or cell-bound antigens
  • T cells recognise cell-bound antigens, peptides from foreign antigens only bound to MHC
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10
Q

What is the mechanism of T cell antigen recognition?

A
  • T cells recognise antigens processed + presented by APC
  • APCs process antigens to peptides (for most T cells)
  • Peptides bind to MHC molecules
  • peptide-MHC complexes are presented on APC surface
  • -> activation of T cells specific for antigenic peptide
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11
Q

What is the most common type of T Cell Receptor?

A

alpha-beta TCR

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12
Q

What is meant by MHC restriction of antigen recognition by T cells?

A
  • CD4+ helper T cells: antigens (peptides) displayed by MHC class II
  • CD8+ cytotoxic T cells: antigens (peptides) displayed by MHC class I

T cells cannot recognise peptides unless they are combined with MHC molecules. MHC restricts antigen recognition by T cells

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13
Q

How does the T cell receptor actually bind to MHC?

A
  • MHC has polymorphic residue which binds to T cell receptor
  • There is a peptide which fits into both the T cell receptor and MHC grooves
    • Anchor residue of peptide fits into “pocket” of MHC
    • T cell contact residue of peptide also
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14
Q

Describe structures of both MHC I and MHC II

A

MHC molecules display peptides from processed antigens

  • MHC I: a chain + B2-microglobulin ; present peptides to CD8+, short peptides can fit into the groove between a1 + a2 (same chain)
  • MHC II: a chain + B-chain ; present peptides to CD4+, larger peptides can fit into groove between a1 and b1 (2 diff chains)
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15
Q

Where are MHC I and MHC II expressed?

A
  • MHC I : all nucleated cells
  • MHC II : antigen presenting cells (dendritic cells, macrophages)
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16
Q

Cells that are important for presenting antigens to CD4 T cells are macrophages and dendritic cells (APCs) - What are key features of dendritic cells?

A
  • sentinel cells
  • skin, mucosa, tissues
  • capture microbes
  • phagocytosis
  • process pathogens + present Ags to T cells
  • link innate + adaptive immune response
17
Q

What are the 3 signals APCs send out for T cell activation?

A
  • Signal 1: antigen recognition -> present antigen on MHC, bind to TCR
  • Signal 2: co-stimulation, upregulation on surface of APCs, ligate receptors on T cells
  • Signal 3: cytokines, produced by APC, tell T cell what kind of effector T cell to differentiate into

All 3 result in T cell activation, all are important as just 1/3 wouldn’t be enough for sustained activation

18
Q

So why does CD4 specifically bind to MHC II and CD8 specifically to MHC I?

A

Eg. You can see that the CD4 T cell forms a complex with the peptide:MHC II structure but also the CD4 directly attaches to a portion of the MHC II molecule.

This is important as in the absence of the CD4 T Cell contacting the MHC II, the T cell will need 100x more MHC:peptide complexes on the surface of the APC in order to get activated. The binding of CD4 to the MHC II allows for a lower threshold for activation.

19
Q

Describe antigen processing and presentation to CD4+ T cells

A

Exogenous antigens (eg bacteria) taken up in cells, processed + presented by MHC II to CD4 T cells

  • uptake of extracellular proteins into vesicular compartments of APC
  • processing of internalised proteins in endosomal/lysosomal vesicles
  • biosynthesis + transport of class II MHC molecules to endosomes
  • association of processed peptides w/ class II MHC molecules in vesicles
  • expression of peptide-MHC complexes on cell surface

tl;dr phagocytosis occurs, then antigen is presented on surface bound to MHC II

20
Q

Describe antigen processing and presentation to CD8+ T Cells

A

Cytosolic antigens (eg virus) are processed + presented by MHC I to CD8+ T cells

  • prod of proteins in cytosol (as opposed to extracellular)
  • proteolytic degradation of proteins
  • transport of peptides from cytosol to ER
  • assembly of peptide-class I complexes in ER
  • surface expression of peptide-MHC class I complexes
21
Q

What is the difference of role between the following types of T cells?

  • Th1
  • Th2
  • CTL
A
  • Th1 (CD4+) - help phagocytes to kill ingested microbes
  • Th2 (CD4+) - help eosinophils/mast cells to kill helminths
  • CTL (CD8+) - kill cells infected by microbes that grow freely in the cytosol
22
Q

What is the main cytokine of Th1-mediated immune responses?

A

IFN-γ (gamma)

23
Q

What is the main role of Th1-mediated responses?

A
  • Activate phagocytosis
  • To increase destruction of intracellular pathogens
  • Stimulate production of IgG from B cells -> inc phagocytosis of microbes (complement + opsonising)
24
Q

Examples of Th1-mediated diseases?

A
  • granulomas
  • autoimmunity
25
Q

What do Th2 cells do?

A
  • Response against infection w/ helminths
    • they are too large to be phagocytosed
    • thick coat: resistant to microbicidal activties of macrophages
  • Th2 cells help B cells prod antibodies that opsonise helminths
  • Antibodies activate eosinophil/mast cell -> destroy helminths
26
Q

What do CD8+ Cytotoxic T lymphocytes do?

A
  • eliminate intracellular microbes (growing in cytosol)
  • kill mostly viruses
  • main mechanism: kill infected cells
  • killing is antigen-specific and contact-dependent
  • healthy cells are not killed by CTLs

Physiology & Science: Immunity Basics (6 decks)

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